The complexity and precision requirements of mechanical products require improved methods of determining, producing and controlling the surface finish of both cast and machined items. Furthermore, increased production efficiency requirements and increasing rework costs demand accurate and consistent assessment of product quality. Surface roughness, or texture, is an important characteristic of both cast and machined products that describes the extent and average size of surface irregularities. Commonly expressed in micro inches, surface roughness is a measure of the arithmetical average deviation of surface irregularities from a hypothetical perfectly smooth surface and is typically denoted by .mu.AA. Increasing manpower costs, increasingly competitive commercial environments and other cost saving measures, are challenging many traditional methods of evaluating surface roughness.
When a precise surface roughness measurement is required, the physical dimensions of surface irregularities can be measured using a profilometer, a device in which the deflection of a diamond tipped stylus is measured as it moves over a surface. The profilometer provides very accurate measurement of surface feature height, but only along a narrow sample of the surface. Thus, many measurements must be taken and averaged together to provide an assessment of surface roughness over an area. Such a procedure can be very time consuming and consequently prohibitively expensive in mass production applications. Finally, any physical contact device is impractical or impossible to use in applications where the product is made of soft materials that might be damaged by the stylus or where the geometry of the product prohibits its use.
Although surface roughness can be precisely measured using a profilometer, it is more common in many applications to determine surface texture by visually comparing the product surface with a known surface roughness standard. A commonly used standard is the Fowler Casting 52-725-334, which has casting samples of roughness measures from 32 to 2000 .mu.AA. Visual comparison of a manufactured surface to a surface standard is a subjective human operation requiring trained experts to assess surface roughness. This approach produces inconsistent results that are dependent upon the experience, knowledge, fatigue level, etc., of the expert.
To save money, surface roughness evaluation often involves only a statistical sampling of a number of parts from a particular lot of manufactured parts. These samples are visually inspected by experts who have been trained to assess the surface finish and the occurrence of various abnormalities, such as porosity and voids, in the exposed surfaces. Such visual inspection is time consuming and can require destruction of the part in order for the inspector to view all critical surfaces. In addition, statistical sampling of casted lots does not provide a representative measure of an entire lot because there is no correlation between the casting process and various molds. Therefore, statistical independence can only be assumed as numerous external factors that affect the quality of the casting have not yet been controlled in such a fashion as to be correlated among the many separate molds that might be used in the manufacturing process. The inability to statistically predict the production quality of castings precipitates the requirement of 100% testing thereby ruling out any destructive test processes.
Surface roughness measurement devices based on the physical theories of light relating the angular distribution of scattered light to the topography of the surface (diffraction theory) do exist. However, this approach is useful only where the average roughness of the surface is much less than the wavelength of the incident radiation. Other optical and image processing methods that correlate the angular diffraction of scattered light and polarity changes of incident light to surface topography have been proposed but require the use of large and expensive optics.